Power systems with inverter input voltage control

ABSTRACT

A direct current (DC) bus voltage from a combined output of a plurality of DC power modules is controlled based on an alternating current (AC) voltage of a power grid. The DC bus voltage tracks the AC grid voltage to provide efficient conversion between the DC power sources and the AC grid, even when the amplitude of the AC grid voltage varies. In one example, a variable reference voltage is generated based on a detected AC grid voltage. The reference voltage increases and decreases in proportion to increases and decreases in the AC grid voltage. In this manner, large differences between the bus voltage and the grid voltage are avoided. By closely tracking the two voltages, efficiency in the modulation index for power conversion can be achieved.

BACKGROUND OF THE INVENTION

Modern power systems may include many types of differing power sources.For example, direct current (DC) power systems are commonly integratedinto traditional alternating current (AC) power grids to providesupplemental power to or otherwise interface with the power grid. Thesesystems may exist in large-scale implementations by commercial powerproviders or small-scale implementations by businesses and residentialusers.

Solar panels are a common type of power source that generates directcurrent (DC) voltages, although other types of DC power sources such aselectrochemical power sources also exist. Typically, the DC voltage andDC current from one solar panel is well below the needs of the voltageand current needed for the Alternating Current (AC) power grid.Typically, many solar panels are used in combination to provide thenecessary voltage and current for the AC power grid. The DCvoltages/currents also need to be converted to AC voltages/currents. Asignificant challenge is to efficiently transfer the DC power from eachsolar panel to the AC power grid. Note that this problem is not limitedto converting from DC power to AC power. Even if the solar panels (orother DC power sources) were to be combined to provide DC power, thereare challenges in efficiently “combining” the DC power. A significantfactor in this challenge is that the power/voltage/current output of asolar panel is not fixed.

SUMMARY

According to one aspect of the present disclosure, there is provided anapparatus that includes a plurality of direct current (DC) power modulesincluding at least a first power module having a first output and asecond power module having a second output; a bus connecting theplurality of DC power modules, the bus connecting the first output andthe second output in series; an inverter having a direct current (DC)terminal and an alternating current (AC) terminal; and a managementcircuit configured to determine an AC voltage at the AC terminal andcontrol a direct current (DC) voltage at the DC terminal based on the ACvoltage.

Optionally, in any of the preceding aspects, the management circuit isconfigured to generate a DC reference voltage based on the AC voltage;and the management circuit is configured to control the DC voltage basedon the DC reference voltage.

Optionally, in any of the preceding aspects, the management circuit isconfigured to control the DC input voltage by varying an amount ofcurrent provided to the AC output based on the AC output voltage.

Optionally, in any of the preceding aspects, the management circuit isconfigured to provide the DC voltage at a first voltage level inresponse to a first magnitude of the AC voltage and to provide the DCvoltage at a second voltage level in response to a second magnitude ofthe AC voltage; the first voltage level of the DC voltage is higher thanthe second voltage level of the DC voltage; and the first magnitude ofthe AC voltage is higher than the second magnitude of the AC voltage.

Optionally, in any of the preceding aspects, the DC voltage is acombined output voltage of the plurality of DC power modules, thecombined output voltage corresponding to a sum of individual outputvoltages of the plurality of DC power modules.

Optionally, in any of the preceding aspects, the management circuit isconfigured to detect the first magnitude and the second magnitude of theAC voltage at the AC terminal.

Optionally, in any of the preceding aspects, the management circuitincludes a controller coupled to the bus; the controller is configuredto generate reference voltage at a first reference level by filteringbased on the first magnitude of the AC voltage and a second level byfiltering based on the second magnitude of the AC voltage; and thecontroller is configured to determine a first amount of referencecurrent based on combining the first reference level and the firstvoltage level and is configured to determine a second amount ofreference current based on combining the second reference level and thesecond voltage level.

Optionally, in any of the preceding aspects, the DC terminal of theinverter is connected to the bus and is configured to receive the DCvoltage; and the AC terminal of the inverter is connected to a powergrid that provides the AC voltage.

Optionally, in any of the preceding aspects, each DC power modulecomprises a DC power source coupled to a DC power converter having anoutput, the outputs of the DC power converters being coupled in series.

Optionally, in any of the preceding aspects, the plurality of powermodules is a plurality of photovoltaic power modules, each power moduleof the plurality having a plurality of photovoltaic cells.

Optionally, in any of the preceding aspects, the plurality of powermodules is a plurality of electrochemical power storage modules, eachpower module of the plurality having a plurality of electrochemicalcells.

According to one aspect of the present disclosure, there is provided acomputer-implemented method, comprising: receiving at a direct current(DC) terminal a combined output voltage of a plurality of DC powermodules connected in series; generating at an alternating current (AC)terminal an alternating current for a power grid, the alternatingcurrent is generated based on the combined output voltage of the DCpower modules; detecting at the AC terminal an AC voltage of the powergrid having a variable amplitude; and generating a variable referencevoltage to control a level of the combined output voltage based on thevariable amplitude of the AC voltage.

Optionally, in any of the preceding aspects, generating a variablereference voltage comprises: generating the variable reference voltageat a first reference level based on a first amplitude of the AC voltage;and generating the variable reference voltage at a second referencelevel based on a second amplitude of the AC voltage; wherein the firstamplitude is greater than the second amplitude; and wherein the firstreference level of the variable reference voltage is higher than thesecond reference level of the variable reference voltage.

Optionally, in any of the preceding aspects, the method furthercomprises generating the combined output voltage at a first voltagelevel in response to the first reference level of the variable referencevoltage; and generating the combined output voltage at a second voltagelevel in response to the second reference level of the variablereference voltage; wherein the first voltage level of the combinedoutput voltage is greater than the second voltage level of the combinedoutput voltage.

Optionally, in any of the preceding aspects, the method furthercomprises generating a variable reference for an output current exportedto the AC output based on the variable amplitude of the AC voltage.

Optionally, in any of the preceding aspects, the combined output voltageof the plurality of power modules is a sum of individual DC outputvoltages of the plurality of power modules.

Optionally, in any of the preceding aspects, the plurality of powermodules is a plurality of photovoltaic power modules; and each powermodule includes a power optimizer and a photovoltaic panel having aplurality of photovoltaic cells.

Optionally, in any of the preceding aspects, the power optimizer of eachpower module includes a DC power converter.

Optionally, in any of the preceding aspects, the variable amplitude is avariable peak amplitude of the AC voltage.

According to one aspect of the present disclosure, there is provided anon-transitory computer-readable medium storing computer instructionsfor controlling a direct current (DC) source, that when executed by oneor more processors, cause the one or more processors to perform thesteps of: detect an alternating current (AC) voltage at an AC terminalof an inverter coupled to a power grid; determine for a bus connected toa DC terminal of the inverter a reference voltage based on the ACvoltage at the AC terminal, the bus connecting outputs of a plurality ofDC power modules in series; and generate one or more indications of thereference voltage in order to provide a bus voltage at a voltage levelbased on a magnitude of the AC voltage at the AC terminal of theinverter.

Optionally, in any of the preceding aspects, the step of detect an ACvoltage comprises detecting a first magnitude of the AC voltage and asecond magnitude of the AC voltage at the AC terminal of the inverter;the step of determine a reference voltage comprises determining a firstreference level for the reference voltage and a second reference levelfor the reference voltage based on the first magnitude of the AC voltageand the second magnitude of the AC voltage; and the step of generate oneor more indications of the reference voltage comprises generating afirst indication of the first reference level of the reference voltageto provide the bus voltage at a first DC voltage level and generating asecond indication of the second reference level of the reference voltageto provide the bus voltage at a second DC voltage level.

Optionally, in any of the preceding aspects, generating one or moreindications of the DC reference voltage comprises: generating thereference voltage at the first reference level; and generating thereference voltage at the second reference level.

Optionally, in any of the preceding aspects, wherein the steps includethe steps of: generate a first amount of reference current based on theindication of the first reference level of the reference voltage;generate a second amount of reference current based on the indication ofthe second reference level of the reference voltage; and generate thebus voltage at the first DC voltage level by providing to the power grida first amount of AC output current based on the first amount ofreference current and generating the bus voltage at the second DCvoltage level by providing to the power grid a second amount of ACoutput current based on the second amount of the reference current;wherein the first reference level of the reference voltage is higherthan the second reference level of the reference voltage; and whereinthe first amount of AC output current is greater than the second amountof AC output current.

According to one aspect of the present disclosure, there is provided anapparatus, comprising: means for receiving at a direct current (DC)terminal a combined output voltage of a plurality of DC power modulesconnected in series; means for generating at an alternating current (AC)terminal an alternating current for a power grid, the alternatingcurrent is generated based on the combined output voltage of the DCpower modules; means for detecting at the AC terminal an AC voltage ofthe power grid having a variable amplitude; and means for generating avariable reference voltage to control a level of the combined outputvoltage based on the variable amplitude of the AC voltage.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter. The claimed subject matter is not limited to implementationsthat solve any or all disadvantages noted in the Background.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting an example of a power system thatincludes a plurality of power modules.

FIG. 2 is a block diagram depicting an example of a power system thatincludes a plurality of power modules having power cells connected inseries.

FIGS. 3A-3B are block diagrams depicting operation of a power systemwith multiple photovoltaic modules during different conditions.

FIG. 4 is a block diagram depicting an example of power system operationby controlling an input bus voltage based on a constant referencevoltage.

FIG. 5 is a graph depicting an alternating current (AC) output voltage,direct current input voltage, and constant reference voltage.

FIG. 6 is block diagram of a power system that provides control of aninput bus voltage based on and output grid voltage in one embodiment.

FIG. 7 is a graph depicting an alternating current (AC) output voltage,direct current input voltage, and variable reference voltage in oneembodiment.

FIG. 8 is a flowchart describing a process for controlling an input busvoltage based on an output voltage in one embodiment.

FIG. 9 is a flowchart describing a process for detecting an outputvoltage at a bus control circuit in one embodiment.

FIG. 10 is a block diagram of a controller configured to control aninput bus voltage based on an output grid voltage in one embodiment.

FIG. 11 is a flowchart describing a process for controlling an input busvoltage using a reference current in one embodiment.

FIG. 12 is a block diagram of a controller in one embodiment.

FIG. 13 is block diagram of a power system including a controllerimplemented separately from a bus control circuit in one embodiment.

FIG. 14 is a block diagram of a computing system such as a controller.

DETAILED DESCRIPTION

The disclosure relates to technology for power system control, such ascontrolling the delivery of power, voltage, and/or current from acombination of direct current (DC) power sources. For example, thetechnology may be used in the control of DC sources such as photovoltaic(e.g., solar) cells, electrochemical cells, and DC batteries. In oneembodiment, the technology may be used in the conversion of power fromdirect current (DC) power sources to an alternating current (AC) powerrequirement of a power grid or other load. For example, the disclosedtechnology provides for the control of a DC input voltage based on amagnitude of an AC output voltage of an inverter.

In one embodiment, a system includes multiple power modules thatindividually generate DC electrical outputs, which are connected inseries by a bus (also referred to as a “string bus”). In thisarrangement, the output voltage of the bus is the sum of individualoutput voltages of the power modules connected by the bus. Such busesmay be used in photovoltaic systems to combine outputs of multiplephotovoltaic modules to achieve a desired output voltage that isprovided to an inverter. The inverter then converts this DC input fromthe bus into an AC output. The voltage provided to an inverter may bemaintained within an acceptable range to provide an adequate level ofinverter efficiency. For example, a bus may provide electrical power toan inverter in a range of 200-500 volts in some situations. Inverterinput voltage may be maintained at some higher voltage than the outputvoltage for efficient inverter operation so that, to produce a 230voltAC output from an inverter (i.e. 230 volts root mean square voltage, or325 volts peak-to-peak), it may be efficient to maintain DC input in arange of about 390 volts (e.g. 380-400 volts, or 350-440 volts).

Photovoltaic modules may generate a current and voltage that dependprimarily on the intensity and wavelengths of sunlight which is notconstant (e.g., time of day, time of year, temperature, cloud cover, andother factors may affect photovoltaic module output). As a result,photovoltaic modules may generate a varying amount of power and mayprovide outputs that vary in DC voltage and/or current. Maintainingefficiency when combining outputs from such variable power sources ischallenging. For example, maintaining an output voltage from a bus to aninverter within a certain range (e.g. within safety limits and/orefficient operating range of inverter) may be challenging when theoutput voltage from the bus is the sum of individual photovoltaic moduleoutputs that fluctuate in sometimes unpredictable ways. For powermodules other than photovoltaic modules, other factors may causefluctuation and may make control challenging.

In addition to variances in the outputs of power modules, the voltage ofa power grid or other load connected to a group of power modules mayvary. For example, some power grids can be expected to fluctuate or havea voltage variance of about 10%. Such variances in the voltage of thepower grid coupled to a DC power system presents challenges inefficiently converting between DC power sources and an AC power grid.For example, variances may lead to decreases in the modulation index ofthe power conversion, which represents the ratio of the AC voltage peakto the DC bus voltage.

In one embodiment, a DC terminal of an inverter coupled to a bus iscontrolled based on an AC terminal of the inverter connected to a powergrid or load. The DC terminal is controlled to track the AC terminal toprovide efficient conversion between the DC power sources and the ACpower grid, even when the amplitude of the AC grid voltage varies. Inone example, a variable reference voltage is generated based on adetected AC voltage at the AC terminal which corresponds to the gridvoltage. The detected AC voltage is the voltage of the load or powergrid connected to the power sources via the inverter. The referencevoltage increases and decreases in proportion to increases and decreasesin the AC terminal voltage in one embodiment. In this manner, largedifferences between the bus voltage and the grid voltage are avoided. Byclosely tracking the two voltages, a high modulation index for powerconversion can be achieved, which results in high efficiency.

FIG. 1 is a block diagram depicting an example of a power system 100that includes a plurality of power modules 104. In this example, eachpower module is a photovoltaic module 104. Each photovoltaic module 104includes a photovoltaic panel (PV) 106 and a respective module outputcircuit 108 that is configured to control the output of the photovoltaicmodule in which it is located. Module output circuits 104 receive inputsfrom photovoltaic panels in their respective modules and provide outputsto a bus 110. A photovoltaic panel may also be referred to herein as asolar panel. Each module output circuit 104 converts DC voltage/currentfrom one or more solar panels to a DC output voltage that is provided tobus 110. Module output circuits 104 include DC-to-DC conversion circuitsthat may include boost-buck circuits, buck-boost circuits, otherswitched-mode power circuits, or other types of DC-to-DC power controlcircuits, including linear power circuits. A DC-to-DC conversion circuitmay be referred to as a power optimizer due to the control mechanismsthe circuit provides to optimize the transfer of DC power from the DCsource to the bus. Bus 110 provides a DC input to inverter 112, whichgenerates an AC output. The AC output is then passed through a meter 114to a power grid (e.g. commercial utility power grid) 120 fordistribution to electrical consumers. The electrical consumers provide aload 122 for the electrical grid which may pass through a meter 116and/or switch 118. In other examples, an inverter output may be usedwithout going through a commercial grid (e.g. photovoltaic modules mayprovide power that is exclusively consumed at a single location, forexample, the house or other building where the photovoltaic modules arelocated). Where the AC output is not supplied to a grid, a meter may beunnecessary and an inverter may directly supply AC power to a load. Forexample, in a domestic photovoltaic system, an inverter may provide anAC output that supplies power to the home. Examples below may beapplicable to various power module configurations and are not limited tophotovoltaic modules connected to a grid or any other specificconfiguration.

FIG. 2 is a schematic illustration of a system 200 that includesphotovoltaic modules 104 each having a respective module output circuit108. It will be understood that the number of modules, n, may be anysuitable number depending on a desired bus voltage and/or other factors.In a given photovoltaic module, photovoltaic cells (“PV Cell”) areconnected in series to provide an input to a module output circuit. Forexample, in each photovoltaic module 104, module output circuit 108receives an input from series connected photovoltaic cells 205 andgenerates a DC output that is provided to bus 210.

In other systems, other types of power cells may be connected in seriesto form other power modules. For example, electrochemical cells may besimilarly connected in series in an electrochemical power module orpower storage module. Examples of power cells include photovoltaic cellsand electrochemical cells. Photovoltaic cells convert photons intoelectricity. Photovoltaic cells may convert photons of variouswavelengths including in the visible spectrum and near-visible spectrum(e.g. infra-red radiation).

Bus 210 is formed of conductors, for example, conductive metal wiresthat connect output terminals of photovoltaic modules. Bus 210 connectsphotovoltaic modules 104 in series so that the voltages are addedtogether to produce a combined voltage that is the sum of theirindividual voltage outputs. Specifically, the outputs of the moduleoutput circuits in each PV module are connected in series. For example,the outputs of the power optimizers in each module output circuit may beconnected in series. In FIG. 2, each photovoltaic module has a lowerterminal that is negative (“−”) and an upper terminal that is positive(“+”) with the positive terminal of a lower photovoltaic moduleconnected to the negative terminal of a neighboring photovoltaic moduleabove it. For example, the positive terminal of photovoltaic module 104d is connected to the negative terminal of photovoltaic module 104 b andthe positive terminal of photovoltaic module 104 b is connected to thenegative terminal of photovoltaic module 104 a. The positive terminal ofphotovoltaic module 104 a is connected to a positive input terminal ofinverter 112, while a negative terminal of photovoltaic module 104 n isconnected to a negative input terminal of inverter 112. Thus, inverter112 receives an input that is the sum of all photovoltaic module outputsconnected to bus 210 (minus any resistive loss). For example, if eachphotovoltaic module 104 a-n provides an output voltage of 10 volts, thenthe combined output voltage supplied to inverter 112 by bus 210 would be10×n.

In operation, the contributions of different photovoltaic modules 104a-n may vary for several reasons so that voltage, current, and powerprovided by individual photovoltaic modules 104 a-n to bus 210 may varyand may cause the bus output voltage to inverter 112 to vary. Forexample, external conditions may cause changes in photovoltaic moduleoutputs. Changing weather (including clouds) may affect differentphotovoltaic modules differently especially where photovoltaic modulesare widely dispersed. Shadows may pass across various photovoltaicmodules at various times causing individual photovoltaic modules toprovide different outputs at different times. Photovoltaic modules mayhave different orientations causing them to have different efficienciesat different times of day. Internal factors may also affect photovoltaicmodules in nonuniform ways. Photovoltaic modules may also providedifferent contributions because of factors that are internal to theindividual photovoltaic modules. For example, photovoltaic modules maywear out at different rates so that some photovoltaic modules may bebecome less efficient than others over time and may be more limited inthe voltage, current, and/or power that they contribute. New replacementphotovoltaic modules may be more efficient than older photovoltaicmodules. Internal faults may cause photovoltaic modules to providereduced output either temporarily or permanently. Other factors may alsocause variation in photovoltaic module output voltage, current, and/orpower.

FIGS. 3A-3B illustrate how changing conditions may affect a photovoltaicsystem 300 in which outputs of multiple photovoltaic module outputcircuits 108 are connected in series by a bus 210. FIG. 3A shows ascenario where all outputs are equal, with each photovoltaic panel “PV”producing a hundred watts (100 W) of power and each module outputcircuits 108 providing this 100 W power at an output voltage of twentyvolts (20v) and a current of five amps (5.0 A) on bus 210. The combinedoutput voltage Vbus of the power modules is 80V and the current throughthe modules and bus is 5.0 A.

FIG. 3B shows a scenario in which the power output of one photovoltaicpanel is reduced to twenty watts (20 W). In this example, a cloud may beshadowing the photovoltaic panel in module 104 c for example. However itwill be understood than several different factors may cause one or morephotovoltaic panels to produce lower than usual power. Module outputcircuit 108 at module 104 c provides a reduced output voltage of fourpoint five volts (5.0v) and a current of four point five amps (4.0 A) asa result of the reduced power. Module output circuits at the othermodules produce increased voltages of twenty-five volts (25v) in thisexample (numbers are rounded off). In some cases, the photovoltaicmodule output voltages shown in FIG. 3B may maintain a bus outputvoltage to an inverter (not shown in FIG. 3B) within a specified range.In other cases, additional adjustment may be required to keep bus outputvoltage within a specified range. For example, with four modulesconnected to a bus in this scenario the output voltage remains(3×25)+5.0V=80V. Additional adjustment to output voltages of moduleoutput circuits may be desirable to bring an output voltage of bus 210back to an acceptable range (e.g. back within a range of 75-85v). Ifthere are more modules, then this adjustment may be sufficient, i.e. ifeach unaffected module increases its output voltage by up to two volts(2v) then in a system with eight or more unaffected modules, a drop ofsixteen volts in an affected module may be made up by the unaffectedmodules.

An inverter and/or a controller typically controls the DC power systemto manage variations in the contributions of individual power modules.FIG. 4 is a block diagram of a power system depicting a plurality ofstrings 354 a-n of PV modules and an inverter 112. The power moduleswithin each string are connected in series as described in FIG. 2. Thevoltages of the modules within the strings are added to produce acombined voltage that is the sum of the modules within the string. Thestrings are connected in parallel so that the combined bus outputvoltage Vbus is equal to the voltage of an individual string.

In one example, inverter 112 controls a bus output of bus 210 within aspecified range. For example, inverter 112 may provide a constant busreference voltage that is used to regulate the bus output voltage.Inverter 112 controls the bus output voltage to follow or be inproportion to the bus reference voltage. In this manner, inverter 112controls the input voltage by controlling the bus output voltage Vbus.By tracking the substantially constant bus reference voltage, the busoutput voltage will be substantially constant. Various techniques may beused to control the combined bus output voltage. In one example,inverter 112 can modify an output current exported to an AC power grid.By varying the output current of the inverter, the bus voltage to theinverter can be regulated. In another example, inverter 112 can providea control signal to the module output circuits 108 of each module. Themodule output circuits can control their individual output voltages tocontrol the bus output voltage.

FIG. 5 depicts a graph 450 illustrating control of a DC bus voltage foran inverter exporting power to an AC power grid using a constantreference voltage. Line 452 depicts the alternating current (AC) voltageat the AC terminal of the inverter, equal to the voltage of an AC powergrid for example. Line 454 depicts a reference voltage Vbus_ref for thebus coupled to the inverter. Line 456 depicts the DC voltage Vbus fromthe bus at the DC terminal of the inverter.

Line 452 illustrates an example of conditions where the voltage Vgrid ofthe power grid has a magnitude and amplitude that varies over time.Vgrid has a peak magnitude of about 350V at a first time at around 0.01s. At time 0.35 s, however, the peak magnitude of the power grid voltagedips to about 300V.

Line 454 depicts a traditional reference voltage Vbus_ref that may beused to control the input bus voltage to the inverter. Vbus_ref has aconstant voltage of about 350V throughout the time from 0.00 s to 0.40s. A constant Vbus_ref is typically used so that the bus voltage Vbus ofthe combined output of the power modules remains constant over time.Individual power modules may adjust their output power, includingvoltage and/or current, to maintain a substantially constant Vbus asconditions vary.

Line 456 depicts the DC voltage Vbus that results from the referencevoltage Vbus_ref. The voltage Vbus remains substantially constant bytracking the constant reference voltage. In this example, Vbus has an ACamplitude of about 20V, centered at 350V.

The ripple in the bus voltage Vbus is provided for exemplary purposesand may not be present in other examples, such as in a balanced threephase inverter system. In a single-phase inverter, for example, a largecapacitor may be used to store power as part of converting between theDC power and AC power. The large capacitance may lead to a smallsinusoidal ripple in the Vbus voltage as shown. In a three-phaseinverter, however, a large capacitor may not be needed such that asubstantial ripple is not present.

The use of a constant reference voltage Vbus_ref may result in aninefficient use of the combined DC voltage of the power modules. Forexample, when the grid voltage dips as shown in FIG. 5, the bus voltageVbus remains the same. In this example, a 50V difference exists betweenthe AC grid voltage peak magnitude and the level of the DC bus voltageVbus. The difference in voltage can lead to poor utilization of the DCbus voltage. Specifically, the modulation index of the inverter isdecreased as the power grid voltage Vgrid decreases relative to theinput voltage Vbus, which may lead to lower efficiency of the inverter.

FIG. 6 is a block diagram depicting a power system 500 in accordancewith one embodiment that provides a variable DC bus voltage to track anAC voltage of a power grid. System 500 includes a plurality of strings354 a-354 n of power modules as previously described. Any number ofstrings may be used, connecting individual power modules in serieswithin a string and the strings in parallel.

A bus control circuit 502 is connected at a DC terminal to bus 210 whichconnects the outputs of the power modules within each string in series.Bus control circuit 502 includes a DC terminal 514 which connects to thebus and receives the combined output voltage Vbus. Bus control circuit502 includes an AC terminal 516 which connects to a power grid having avoltage Vgrid. Bus control circuit 502 may include an inverter 512 and acontroller 504 configured to control conversion of the input DC power tooutput AC power by the inverter.

In accordance with one embodiment, bus control circuit 502 includes avoltage controller 504 that is configured to control the voltage Vbus atthe DC terminal of the bus control circuit based on the AC terminalvoltage of the bus control circuit. Controller 504 detects the voltageVgrid at the AC terminal of inverter 512 and controls the voltage Vbusbased on the voltage Vgrid. In one embodiment, the controller generatesa variable reference voltage Vbus_ref to have a voltage level that isbased on the magnitude or amplitude of the voltage Vgrid. The referencevoltage is substantially proportional to the voltage Vgrid. In thismanner, the level of the reference voltage increases or decreases inproportion to increases and decreases in the AC terminal voltage.Controller 504 in turn controls Vbus based on the reference voltageVbus_ref. Accordingly, the DC terminal voltage Vbus varies in proportionto changes in the AC terminal voltage Vgrid.

Controlling the voltage Vbus based on the voltage Vgrid can lead toincreased efficiency in transferring power form the DC power modules tothe AC power grid. The input voltage Vbus, corresponding to the combinedoutput voltage of the power modules in a string, is tightly coupled tothe power grid voltage Vgrid to maintain a high modulation index forpower conversion. In this manner, losses in the inverter powerconversion can be minimized.

In one embodiment, bus control circuit 502 includes an inverter 512 andcontroller 504 for controlling power conversion by the inverter.Controller 504 may include a processor, microprocessor, or multipleprocessing devices providing a control stage for the inverter. In oneembodiment, controller 504 includes a voltage control circuit. Any oneor any combination of bus control circuit 502, including inverter 110and controller 504, and module output circuits 108 may be referred to asmanagement circuitry. Management circuitry may also or alternativelyrefer to a controller external to bus control circuit 502 as hereinafterdescribed. The management circuitry performs the functions describedherein. The management circuitry may include a controller such as amicrocontroller in one example. The controller may comprise a processor,ROM, RAM, and a memory interface. The storage devices (e.g., ROM, RAM)comprise code such as a set of instructions, and the processor isoperable to execute the set of instructions to provide the functionalitydescribed herein. A memory interface in communication with storagedevices and a processor is an electrical circuit that provides anelectrical interface for a controller. Bus control circuit 502 is oneexample of a means for generating at an alternating current (AC)terminal an alternating current for a power grid, where the alternatingcurrent is generated based on the combined output voltage of a pluralityof DC power modules.

FIG. 7 depicts a graph 460 illustrating control of a DC voltage from abus for a plurality of power modules based on an AC voltage of a powergrid or other load. Line 462 depicts the alternating current (AC)voltage at the AC terminal of the inverter, equal to the voltage of theAC power grid. Line 464 depicts a reference voltage Vbus_ref for the buscoupled to the DC terminal of the inverter and line 466 depicts the busvoltage Vbus of the bus.

As earlier described, the peak magnitude of the power grid voltage fallsby about 50V during the time period depicted in FIG. 7. Specifically,the peak magnitude falls from about 350V to about 300V over the timeperiod between 0.175 s to 0.225 s as the amplitude decreases.

In order to maintain a high modulation index for DC to AC powerconversion, a variable reference voltage Vbus_ref shown by line 466 isprovided. As shown, the reference voltage is reduced beginning at 0.20s. The reference voltage is about 350V at 0.20 s. The voltage is loweredto about 310V at 0.25 s. The reference voltage remains at 310V while theoutput voltage remains at 300V.

The resulting bus voltage Vbus that is provided to the DC terminal ofthe bus control circuit tracks the reference voltage Vbus_ref as shownby line 464. In this manner, the DC terminal voltage is tightly coupledto the AC voltage of the power grid. A large difference between the ACterminal voltage and the DC terminal voltage is avoided to maintain ahigh modulation index for power conversion.

FIG. 8 is a flowchart describing a process 500 of controlling DC powerconversion to an AC power grid based on the amplitude of the AC voltageof the power grid. Process 500 may be practiced in the environment ofFIG. 6 using a controller to control the DC voltage provided to buscontrol circuit 502, but is not limited to such an example of a powersystem environment. For example, process 500 may be practiced by amanagement circuit that is implemented separately from bus controlcircuit 502.

At step 502, the voltage Vgrid at the AC terminal of the bus controlcircuit 502 is detected. In one embodiment, a management circuit such asvoltage controller 504 detects the voltage at the inverter of the buscontrol circuit. In another example, the management circuit may receivean indication of the grid voltage from a sensor, device, or othermodule. In yet another example, a management circuit may detect orreceive an indication of the grid voltage at step 502. An indication ofthe grid voltage may be the grid voltage itself in one example. Step 502may include determining the magnitude or amplitude of the AC voltage.

At step 504, a reference voltage Vbus_ref for the bus connecting thepower modules is calculated based on the magnitude of the grid voltageVgrid. The reference voltage is calculated by determining the peakamplitude of the grid voltage in one embodiment. Additionally, filteringand/or scaling of the grid voltage may be performed in order todetermine a reference voltage suitable for a particular implementation.Calculating the reference voltage may comprise calculating a voltagelevel for the reference voltage.

At step 506, the voltage at the DC terminal of the bus control circuitis controlled based on the calculated reference voltage. Step 506 caninclude varying the input voltage Vbus from the bus based on thecalculated reference voltage. Step 506 can include tightly coupling theinput voltage Vbus to track the reference voltage Vbus_ref. In oneexample, step 506 includes attempting to reduce an error equal to thedifference between the magnitude of Vbus and Vbus_ref. The level of thereference voltage Vbus_ref can be controlled to thereby control thelevel of the bus voltage Vbus. Step 506 may include controlling analternating current provided from an inverter to a power grid in orderto control the bus voltage Vbus at a DC terminal of the inverter.

FIG. 9 is a flowchart describing a process 510 of detecting analternating current voltage at an AC terminal for a bus control circuithaving a direct current DC terminal coupled to a plurality of powermodules. In one example, process 510 may be used at step 502 of FIG. 8.Process 510 describes determining the voltage at the AC terminal of aninverter coupled to a power grid. In one example, the AC terminalvoltage is the grid voltage which can be detected as part of generatingan AC current to be injected into the power grid. Process 510 may bepracticed in the environment of FIG. 6 using a controller to control thevoltage at the DC terminal of the bus control circuit 502, but is notlimited to such an example of a power system environment. For example,process 510 may be practiced by a management circuit that is implementedseparately from bus control circuit 502.

At step 514, a direct current input is received from a bus. In oneexample, the bus connects a plurality of power modules in series andprovides a bus voltage Vbus equal to a sum of the output voltagesproduced by a group of series-connected power modules. The DC input isthe combined output voltage Vbus in one embodiment. At step 514, the DCinput is converted into an AC output for a power grid or other load.Standard voltage conversion can be used to by an inverter to generatefrom a direct current signal an alternating current signal having asinusoidal shape.

At step 516, the voltage Vgrid at the AC terminal of the inverter isdetermined. In one embodiment, the peak amplitude of the AC voltage atthe AC terminal of the inverter is determined at step 516. The ACvoltage can be calculated or measured at step 516. In one embodiment,determining the peak amplitude comprises determining the peak magnitudeof the absolute value of the AC voltage An absolute value of the ACterminal voltage can be taken, followed by determining the peak value ofthe absolute value. In one embodiment, step 516 includes generating anindication of the AC voltage. The indication can be used by controller504 to generate a bus reference voltage. In another example, theindication can be used by a management circuit separate from the buscontrol circuit. The bus reference voltage may be used to generate areference current. The reference current can be used to control theamount of current injected to the power grid by the inverter.

FIG. 10 is a block diagram of a portion of a controller 600 in oneembodiment. FIG. 10 is one example of a controller as shown in FIG. 6that is used to control a DC bus input based on an AC output. Controller600 can be implemented using hardware, software, and/or firmware.

Controller 600 receives the voltage Vgrid of the power grid at an ACterminal. An indication of the grid voltage or the actual grid voltagemay be received. The grid voltage is passed to an absolute value unit602 that determines an absolute value of the grid voltage. As shown inFIG. 6, for example, the grid voltage is an AC voltage having asinusoidal shape that oscillates between positive and negative levels.The absolute value module provides an absolute value of the varyingsignal of the grid.

The absolute values are then passed through a peak calculation unit 604that determines a peak amplitude or magnitude of the grid voltage. Thepeak amplitude is then passed through a filter unit 606. The filter unit606 is a low-pass filter in one example that removes spikes or otherextremes in the grid voltage magnitude. The filter unit can be used tocontrol changes to the reference voltage by controller 600. Filteringcan be used to slow the rate of change to the reference and inputvoltage, for example.

The filtered signal is then passed to a scaling unit 608. The scalingunit can be used to increase the value of the reference voltage over thelevel of the power grid. By scaling the grid voltage to calculate thereference voltage, over-modulation and/or other errors can be reduced oravoided. In one example, the scaling unit 608 provides a scaling factorof 1.05, but other factors may be used as suitable for a particularimplementation.

The result of the scaling unit is a voltage level for the referencevoltage Vbus_ref in this example. The reference voltage Vbus_ref is thenused in additional stages to control the bus voltage Vbus by changingthe output current provided to the power grid. The reference voltage ispassed to a subtractor 610 or other combiner. The voltage Vbus at the DCterminal is received at a filter module 612. Filter module 612 canprovide low pass filtering of the Vbus voltage, similar to filter module606. The filtered Vbus voltage is then provided to the subtractor 610.Subtractor 610 combines the reference voltage Vbus_ref and the busvoltage Vbus to generate an error representing the difference betweenthe values. In one example, subtractor 610 subtracts the level of thebus voltage from the level of the reference voltage to determine anamount of error.

The error resulting from the subtractor calculation is then passed to aproportional integral (PI) unit 614. PI unit 614 has the amount of erroras a input and generates a reference current Iref based on the amount oferror. In one embodiment, the PI unit generates a current referenceIref. The reference current Iref is the magnitude of the alternatingcurrent generated by the bus control circuit in one embodiment. Thereference current is an amount of current targeted for injection intothe power grid in one embodiment. PI unit 614 can control the referencecurrent based on the error, which in turn will control the bus voltageVbus. For example, if the error is a positive value indicating the levelof the bus voltage Vbus is higher than the reference voltage Vbus_ref,Iref can be increased. By increasing the amount of output AC currentgenerated by the bus control circuit, the amount of current drawn fromthe DC bus capacitor is increased, which results in a drop in Vbus. Inone embodiment, PI unit 614 selects an amount of reference current Irefbetween a minimum reference current Iref_min and a maximum referencecurrent Iref_max, to avoid an output current exceeding the capacity ofthe inverter. In one embodiment, PI module 614 provides proportionalintegral control to provide changes to the Iref current, therebycontrolling the current drawn from the DC bus capacitor and the busvoltage. While proportional integral control is used in the exampleillustrated here, any suitable form of feedback control may be used.

FIG. 11 is a flowchart describing a process 700 of controlling acombined DC bus voltage Vbus from a plurality of power modules based onan AC voltage of a grid or other load. In one example, process 700 maybe used at step 506 of FIG. 8. Process 700 describes a process ofaccessing an AC voltage at an AC terminal of an inverter, generating areference bus voltage based on the AC voltage, and generating a DC busvoltage that tracks the reference bus voltage. Process 700 may bepracticed in the environment of FIG. 6 using a voltage controller tocontrol the DC input to bus control circuit 502, but is not limited tosuch an example of a power system environment. For example, process 700may be practiced by a management circuit that is implemented separatelyfrom bus control circuit 502.

At step 702, an alternating current (AC) output voltage is received. Inone example, an AC voltage Vgrid for a power grid or other loadconnected to a bus control circuit is received at step 702. In oneembodiment, an indication of the AC voltage is received at step 702. Theindicator can be the actual AC voltage or a representation of thevoltage, such as a digital or analog representation of the AC voltage.

At step 704, an absolute value of the voltage Vgrid is determined. Atstep 706, a peak amplitude of the AC voltage is determined from theabsolute value. In one embodiment, step 706 comprises determining a peakmagnitude of the of the AC voltage. At step 708, the peak amplitude isfiltered to by a low pass filter to remove spikes or other extremes inorder to slow the rates of change to the reference and input voltage.

At step 710, the reference voltage Vbus_ref is calculated. The referencevoltage is calculated by scaling the filtered output signal in oneembodiment. The filtered voltage can be scaled (e.g., by a factor of1.05) to increase the value of the reference voltage over the level ofthe power grid. The result of scaling is the reference voltage Vbus_ref.In one embodiment, step 710 comprises determining a voltage level forthe reference voltage Vbus_ref.

At step 712, the DC bus voltage Vbus is received. At step 714, the busvoltage Vbus is passed through a low pass filter. It is noted that steps712 and 714 may be performed independently and without relation to theorder of steps 702-710. For example, steps 702-710 are performed inparallel in one embodiment. At step 716, an error is calculated based onthe reference voltage Vbus_ref and the input bus voltage Vbus. Inembodiment, a subtractor is used to combine the reference voltageVbus_ref and the bus voltage Vbus to generate an error representing thedifference between the values.

At step 718, a reference current is determined based on the errorbetween the voltages. A PI module uses the error along with otherparameters to generate a reference current in one embodiment usingproportional integral control. An amount of reference current isdetermined in one embodiment.

At step 720, the output AC current is generated based on the referencecurrent at step 718. In one embodiment, the inverter provides an amountor level of output current that is proportional to the amount or levelof the reference current generated in step 718. By controlling thereference current, the bus voltage Vbus is controlled. The amount ofreference current can be increased, thereby increasing the amount ofoutput current exported to the AC grid. By increasing the amount ofcurrent output to the AC grid, the level of the bus voltage will bedecreased. If the reference current is decreased, the output currentexported to the grid is decreased. By decreasing the current output tothe AC grid, the bus voltage will be increased.

FIG. 12 is a block diagram of one embodiment of an apparatus 900 forcontrolling a combined output voltage from a plurality of power modulesto a DC terminal of an inverter based on an AC voltage of a power gridcoupled to an AC terminal of the inverter. In one embodiment, theapparatus 900 may include a controller 504 as shown in FIG. 6. Theapparatus 900 may also include a stand-alone controller or othercomputing device as described above. In certain embodiments, processes500, 510, and 700 may be performed based on apparatus 900. The apparatusincludes a voltage detection unit 902, reference control unit 904, andbus control unit 906. The various units could be implemented with anycombination of hardware and/or software.

In general, the voltage detection unit 902 is configured to determine anAC voltage of a power grid. In one embodiment, the AC voltage of thepower grid is a voltage at an AC terminal of an inverter or othervoltage converter connected to a plurality of DC power modules. Unit 902can be configured to determine an AC voltage of the AC terminal which iscoupled to the power grid. Voltage detection unit 902 is one example ofa voltage detection means for determining or detecting an AC voltage atan AC terminal of an inverter coupled to an AC power grid. Voltagedetection unit 902 is one example of a means for receiving a DC voltageand current from a bus connecting the outputs of a plurality of powermodules. Voltage detection unit 902 is one example of a means receivingat a direct current (DC) terminal a combined output voltage of aplurality of DC power modules connected in series. Voltage detectionunit is one example of a means for detecting at an AC terminal an ACvoltage having a variable amplitude.

The reference control unit 904 is configured to generate one or morereference signals for controlling a DC bus coupled to a DC terminal ofan inverter. Reference control unit 904 can be configured to generate areference voltage for controlling a bus voltage or a reference currentfor controlling the bus voltage. Reference control unit 904 mayalternately or additionally generate one or more indications of areference voltage or current. Reference control unit 904 may control avoltage level of a reference voltage or an amount of reference current.Reference control unit 904 is one example of a reference control meansfor providing a reference DC voltage for a bus connecting the outputs ofa plurality of power modules in series. Reference control unit 904 isone example of a means for generating a variable reference voltage tocontrol a DC input from a bus based on an amplitude of an AC voltage.Reference control unit 904 is one example of a means for generating avariable reference current to control a DC voltage of a bus. Referencecontrol unit 904 is one example of a means for determining a DCreference voltage based on an AC voltage of a power grid and generatingone or more indications of the DC reference voltage.

The bus control unit 906 is configured to control a DC bus voltageprovided to the DC terminal of an inverter from a plurality of powermodules based on an AC voltage of the power grid. The bus control unit906 is configured to control a bus voltage based on an AC voltage in oneexample. The bus control unit 906 controls the bus voltage based on areference voltage or current that tracks an AC voltage of the powergrid. Bus control unit 906 is one example of a means for controlling aDC voltage based on a reference DC voltage that varies with an ACvoltage. Input control unit 906 is one example of a means forcontrolling a DC voltage from a bus based on a variable peak magnitudeof an AC voltage of a power grid.

The units of FIG. 12 may include or be formed as part of any suitableprocessing device. The units of FIG. 12 may include or be formed as partof an inverter or bus control circuit 502. In some example embodiments,the apparatus 900 may further include one or more elements forperforming any one or combination of steps described in the embodiments.In accordance with various embodiments of the present disclosure, themethods described herein may be implemented using a hardware computersystem that executes software programs.

FIG. 13 is a block diagram depicting a power system 800 in accordancewith one embodiment. System 800 includes a plurality of strings 354a-354 n of power modules as previously described in FIG. 6. A buscontrol circuit 502 is connected to bus 210 which connects the outputsof the power modules within each string in series. As earlier described,bus control circuit 502 receives the combined output voltage Vbus at aDC terminal and generates an output AC current that is provided to apower grid having a voltage Vgrid. Bus control circuit 502 may includean inverter and a controller configured to control conversion of theinput DC power to an output AC power.

In accordance with the embodiment shown in FIG. 13, controller 504 isimplemented separately from bus control circuit 502. Controller 504 maybe implemented as an independent circuit in one embodiment. In oneexample, controller 354 is implemented as a stand-alone controller suchas by a processor or microcontroller. Controller 504 may generateindications of the AC voltage of the grid, DC voltage of the bus,reference voltage, and/or reference current. In one embodiment, theindications are the actual voltages and/or currents. In other examples,the indications are representations of the actual voltages and currents.

A communication bus 820 connects controller 504 with bus control circuit820. Controller 504 may provide an indicator of a reference current orvoltage to bus control circuit to be used in controlling the busvoltage. In another example, controller 504 may provide an indication ofthe reference voltage directly to the bus control circuit. The buscontrol circuit can then control the bus voltage.

A communication bus 822 connects controller 504 with module outputcircuits 108. In one embodiment, controller 504 may control moduleoutput circuits 108 in order to coordinate outputs provided to bus 210so that a bus output provided to inverter 512 may be maintained within aspecified range. Controller 504 can be connected to module outputcircuits 104 to control respective outputs of the module outputcircuits. The controller may control these outputs based on a variety offactors including voltage, current, or other measured values at one ormore points in photovoltaic system 200. A controller may provide inputsto particular module output circuits to cause them to change theiroutputs in specified ways. For example, a controller may command amodule output circuit to increase its output voltage or current. In someexamples, a module output circuit may include a switched-mode powercircuit and a controller may command a module output circuit to operatesuch a switched-mode power circuit at a particular modulation index,which may provide a specific output.

In one embodiment, controller 504 includes a communication bus 820 tobus control circuit 502 but not the module output circuits 108. Inanother embodiment, controller 504 includes a communication bus 822 tomodule output circuits 108, but not the bus control circuit 502.

FIG. 14 is a high level block diagram of a computing system 1300 thatcan be used to implement various embodiments. In one example, computingsystem 1300 is an inverter or bus control circuit. Specific devices mayutilize all of the components shown, or only a subset of the components,and levels of integration may vary from device to device. Furthermore, adevice may contain multiple instances of a component, such as multipleprocessing units, processors, memories, transmitters, receivers, etc.

The computing system may comprise a processing unit 1301 equipped withone or more input/output devices, such as network interfaces, storageinterfaces, and the like. The processing unit 1301 may include a centralprocessing unit (CPU) 1310, a memory 1320, a mass storage device 1330,and an I/O interface 1360 connected to a bus. The bus may be one or moreof any type of several bus architectures including a memory bus ormemory controller, a peripheral bus or the like. Processing unit 1301may be used to implement any of the computing devices described herein,such as remote devices 160, and/or hosts 112.

The CPU 1310 may comprise any type of electronic data processor. The CPU1310 may be configured to implement any of the schemes described herein,such as the processes illustrated in FIGS. 8, 9, and 11 using any one orcombination of steps described in the embodiments. The memory 1320 maycomprise any type of system memory such as static random access memory(SRAM), dynamic random access memory (DRAM), synchronous DRAM (SDRAM),read-only memory (ROM), a combination thereof, or the like. In anembodiment, the memory 1320 may include ROM for use at boot-up, and DRAMfor program and data storage for use while executing programs. Inembodiments, the memory 1320 is non-transitory. The mass storage device1330 may comprise any type of storage device configured to store data,programs, and other information and to make the data, programs, andother information accessible via the bus. The mass storage device 1330may comprise, for example, one or more of a solid state drive, hard diskdrive, a magnetic disk drive, an optical disk drive, or the like.

The processing unit 1301 also includes one or more network interfaces1350, which may comprise wired links, such as an Ethernet cable or thelike, and/or wireless links to access nodes or one or more networks1380. The network interface 1350 allows the processing unit 1301 tocommunicate with remote units via the network 1380. For example, thenetwork interface 1350 may provide wireless communication via one ormore transmitters/transmit antennas and one or more receivers/receiveantennas. In an embodiment, the processing unit 1301 is coupled to alocal-area network or a wide-area network for data processing andcommunications with remote devices, such as other processing units, theInternet, remote storage facilities, or the like. In one embodiment, thenetwork interface 1350 may be used to receive and/or transmit interestpackets and/or data packets in an ICN. Herein, the term “networkinterface” will be understood to include a port.

The components depicted in the computing system of FIG. 14 are thosetypically found in computing systems suitable for use with thetechnology described herein, and are intended to represent a broadcategory of such computer components that are well known in the art.Many different bus configurations, network platforms, and operatingsystems can be used.

The technology described herein can be implemented using hardware,software, or a combination of both hardware and software. The softwareused is stored on one or more of the processor readable storage devicesdescribed above (e.g., memory 82, mass storage 84 or portable storage92) to program one or more of the processors to perform the functionsdescribed herein. The processor readable storage devices can includecomputer readable media such as volatile and non-volatile media,removable and non-removable media. By way of example, and notlimitation, computer readable media may comprise computer readablestorage media and communication media. Computer readable storage mediais non-transitory and may be implemented in any method or technology forstorage of information such as computer readable instructions, datastructures, program modules or other data. Examples of computer readablestorage media include RAM, ROM, EEPROM, flash memory or other memorytechnology, CD-ROM, digital versatile disks (DVD) or other optical diskstorage, magnetic cassettes, magnetic tape, magnetic disk storage orother magnetic storage devices, or any other medium which can be used tostore the desired information and which can be accessed by a computer.Communication media typically embodies computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism and includesany information delivery media. The term “modulated data signal” means asignal that has one or more of its characteristics set or changed insuch a manner as to encode information in the signal. By way of example,and not limitation, communication media includes wired media such as awired network or direct-wired connection, and wireless media such as RFand other wireless media. Combinations of any of the above are alsoincluded within the scope of computer readable media.

A computer-readable non-transitory media includes all types of computerreadable media, including magnetic storage media, optical storage media,and solid state storage media and specifically excludes signals. Itshould be understood that the software can be installed in and sold witha device. Alternatively the software can be obtained and loaded into adevice, including obtaining the software via a disc medium or from anymanner of network or distribution system, including, for example, from aserver owned by the software creator or from a server not owned but usedby the software creator. The software can be stored on a server fordistribution over the Internet, for example.

In alternative embodiments, some or all of the software can be replacedby dedicated hardware including custom integrated circuits, gate arrays,FPGAs, PLDs, and special purpose computers. In one embodiment, software(stored on a storage device) implementing one or more embodiments isused to program one or more processors. The one or more processors canbe in communication with one or more computer readable media/ storagedevices, peripherals and/or communication interfaces. In alternativeembodiments, some or all of the software can be replaced by dedicatedhardware including custom integrated circuits, gate arrays, FPGAs, PLDs,and special purpose computers.

The foregoing detailed description has been presented for purposes ofillustration and description. It is not intended to be exhaustive or tolimit the subject matter claimed herein to the precise form(s)disclosed. Many modifications and variations are possible in light ofthe above teachings. The described embodiments were chosen in order tobest explain the principles of the disclosed technology and itspractical application to thereby enable others skilled in the art tobest utilize the technology in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

The disclosure has been described in conjunction with variousembodiments. However, other variations and modifications to thedisclosed embodiments can be understood and effected from a study of thedrawings, the disclosure, and the appended claims, and such variationsand modifications are to be interpreted as being encompassed by theappended claims. In the claims, the word “comprising” does not excludeother elements or steps, and the indefinite article “a” or “an” does notexclude a plurality. A single processor or other unit may fulfill thefunctions of several items recited in the claims. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate, preclude or suggest that a combination of these measurescannot be used to advantage. A computer program may be stored ordistributed on a suitable medium, such as an optical storage medium or asolid-state medium supplied together with, or as part of, otherhardware, but may also be distributed in other forms, such as via theInternet or other wired or wireless telecommunication systems.

What is claimed is:
 1. An apparatus, comprising: a plurality of directcurrent (DC) power modules including at least a first power modulehaving a first output and a second power module having a second output;a bus connecting the plurality of DC power modules, the bus connectingthe first output and the second output in series; an inverter having adirect current (DC) terminal and an alternating current (AC) terminal;and a management circuit configured to determine an AC voltage at the ACterminal and control a direct current (DC) voltage at the DC terminalbased on the AC voltage.
 2. The apparatus of claim 1, wherein: themanagement circuit is configured to generate a DC reference voltagebased on the AC voltage; and the management circuit is configured tocontrol the DC voltage based on the DC reference voltage.
 3. Theapparatus of claim 2, wherein: the management circuit is configured tocontrol the DC voltage by varying an amount of current provided to theAC terminal based on the AC voltage.
 4. The apparatus of claim 3,wherein: the management circuit is configured to provide the DC voltageat a first voltage level in response to a first magnitude of the ACvoltage and to provide the DC voltage at a second voltage level inresponse to a second magnitude of the AC voltage; the first voltagelevel of the DC voltage is higher than the second voltage level of theDC voltage; and the first magnitude of the AC voltage is higher than thesecond magnitude of the AC voltage.
 5. The apparatus of claim 4,wherein: the DC voltage is a combined output voltage of the plurality ofDC power modules, the combined output voltage corresponding to a sum ofindividual output voltages of the plurality of DC power modules.
 6. Theapparatus of claim 5, wherein: the management circuit is configured todetect the first magnitude and the second magnitude of the AC voltage atthe AC terminal.
 7. The apparatus of claim 6, wherein: the managementcircuit includes a controller coupled to the bus; the controller isconfigured to generate a reference voltage at a first reference level byfiltering based on the first magnitude of the AC voltage and a secondlevel by filtering based on the second magnitude of the AC voltage; andthe controller is configured to determine a first amount of referencecurrent based on combining the first reference level and the firstvoltage level and is configured to determine a second amount ofreference current based on combining the second reference level and thesecond voltage level.
 8. The apparatus of claim 1, wherein: the DCterminal of the inverter is connected to the bus and is configured toreceive the DC voltage; and the AC terminal of the inverter is connectedto a power grid that provides the AC voltage.
 9. The apparatus of claim1, wherein: each DC power module comprises a DC power source coupled toa DC power converter having an output, the outputs of the DC powerconverters being coupled in series.
 10. The apparatus of claim 1 whereinthe plurality of power modules is a plurality of photovoltaic powermodules, each power module of the plurality having a plurality ofphotovoltaic cells.
 11. The apparatus of claim 1 wherein the pluralityof power modules is a plurality of electrochemical power storagemodules, each power module of the plurality having a plurality ofelectrochemical cells.
 12. A method, comprising: receiving at a directcurrent (DC) terminal a combined output voltage of a plurality of DCpower modules connected in series; generating at an alternating current(AC) terminal an alternating current for a power grid, the alternatingcurrent is generated based on the combined output voltage of the DCpower modules; detecting at the AC terminal an AC voltage of the powergrid having a variable amplitude; and generating a variable referencevoltage to control a level of the combined output voltage based on thevariable amplitude of the AC voltage.
 13. The method of claim 12,wherein generating a variable reference voltage comprises: generatingthe variable reference voltage at a first reference level based on afirst amplitude of the AC voltage; and generating the variable referencevoltage at a second reference level based on a second amplitude of theAC voltage; wherein the first amplitude is greater than the secondamplitude; and wherein the first reference level of the variablereference voltage is higher than the second reference level of thevariable reference voltage.
 14. The method of claim 13, furthercomprising: generating the combined output voltage at a first voltagelevel in response to the first reference level of the variable referencevoltage; and generating the combined output voltage at a second voltagelevel in response to the second reference level of the variablereference voltage; wherein the first voltage level of the combinedoutput voltage is greater than the second voltage level of the combinedoutput voltage.
 15. The method of claim 12, further comprising:generating a variable reference for an output current exported to thepower grid based on the variable amplitude of the AC voltage.
 16. Themethod of claim 12, wherein: the combined output voltage of theplurality of power modules is a sum of individual DC output voltages ofthe plurality of power modules.
 17. The method of claim 12, wherein: theplurality of power modules is a plurality of photovoltaic power modules;and each power module includes a power optimizer and a photovoltaicpanel having a plurality of photovoltaic cells.
 18. The method of claim17, wherein: the power optimizer of each power module includes a DCpower converter.
 19. The method of claim 12, wherein: the variableamplitude is a variable peak amplitude of the AC voltage.
 20. Anon-transitory computer-readable medium storing computer instructionsfor controlling a direct current (DC) source, that when executed by oneor more processors, cause the one or more processors to perform thesteps of: detect an alternating current (AC) voltage at an AC terminalof an inverter coupled to a power grid; determine for a bus connected toa DC terminal of the inverter a reference voltage based on the ACvoltage at the AC terminal, the bus connecting outputs of a plurality ofDC power modules in series; and generate one or more indications of thereference voltage in order to provide a bus voltage at a voltage levelbased on a magnitude of the AC voltage at the AC terminal of theinverter.
 21. The non-transitory computer-readable medium of claim 20,wherein: the step of detect an AC voltage comprises detecting a firstmagnitude of the AC voltage and a second magnitude of the AC voltage atthe AC terminal of the inverter, the first magnitude is greater than thesecond magnitude; the step of determine a reference voltage comprisesdetermining a first reference level for the reference voltage and asecond reference level for the reference voltage based on the firstmagnitude of the AC voltage and the second magnitude of the AC voltage,the first reference level is greater than the second reference level;and the step of generate one or more indications of the referencevoltage comprises generating a first indication of the first referencelevel of the reference voltage to provide the bus voltage at a first DCvoltage level and generating a second indication of the second referencelevel of the reference voltage to provide the bus voltage at a second DCvoltage level, the first DC voltage level is greater than the second DCvoltage level.
 22. The non-transitory computer-readable medium of claim21, wherein the generating one or more indications of the referencevoltage comprises: generating the reference voltage at the firstreference level; and generating the reference voltage at the secondreference level.
 23. The non-transitory computer-readable medium ofclaim 21, wherein the steps include the steps of: generate a firstcurrent reference based on the indication of the first reference levelof the reference voltage; generate a second current reference based onthe indication of the second reference level of the reference voltage;and generate the bus voltage at the first DC voltage level by providingto the power grid a first amount of AC output current based on the firstcurrent reference and generating the bus voltage at the second DCvoltage level by providing to the power grid a second amount of ACoutput current based on the second current reference; wherein the firstamount of AC output current is less than the second amount of AC outputcurrent.
 24. An apparatus, comprising: means for receiving at a directcurrent (DC) terminal a combined output voltage of a plurality of DCpower modules connected in series; means for generating at analternating current (AC) terminal an alternating current for a powergrid, the alternating current is generated based on the combined outputvoltage of the DC power modules; means for detecting at the AC terminalan AC voltage of the power grid having a variable amplitude; and meansfor generating a variable reference voltage to control a level of thecombined output voltage based on the variable amplitude of the ACvoltage.